Wednesday, 11 April 2018

Define total participation in ER modelling

Define total participation in ER modeling

Participation Constraint

Total participation is one
type of participation constraints. Participation constraints deal with the
participation of entities from an entity set in a relationship set.
Participation constraint is sometimes referred as minimum cardinality
constraint.

Total participation

If each entity of an
entity set must participate in a relationship, then this situation is called as
total participation. And total participation in ERD is shown with single line
(link) between the entity set and the relationship set.

Example
1:

For example, let us consider
a database to manipulate details about cars and its owners. Also, let us assume
some conditions as follows;

An owner must have at
least one car (or more cars)

A car must belong to at
least one owner (or more owners)

For this database we have
two entity sets namely Owner and Car. These two entity sets are linked through a
relationship set Car_Owner which should be a many-to-many relationship
(according to the conditions given). The ER diagram for this database looks
like follows;

In this ERD, Car_Owner is
many-to-many due to the fact that an owner can have one or more cars and a car
is owned by one or more owners. In simpler terms, an owner must own at least one car and a car must have at least one
owner. It leads to have one record for each owner-car combination in
Car_Owner relationship.

In terms of participation
constraints we can say that each Owner entity must participate in the
relationship Car_Owner. Hence, the participation of Owner in Car_Owner is Total.
Also, each Car entity must participate in the relationship Car_Owner. Hence, the
participation of Car in Car_Owner is Total.

Example
2:

Let us consider another
database that stores data about entity sets Owner, Car, and Bike. Also, let us
assume the following conditions;

An owner can have one or
more cars or bikes or both

A car can have one or more
owners

A bike can have one or
more owners

The following ER diagram shows
the database with three entity sets Owner, Car, Bike and two relationship sets Car_Owner
and Bike_Owner for the given requirement.

In this ERD, it is not
compulsory for an owner to have a car or bike. He may have a car or bike or
both. So, there may a owner who has a car but not a bike. That means that owner
entity is participated in Car_Owner relationship but not in Bike_Owner
relationship. Hence, the participation of Owner entity set on either Car_Owner
or Bike_Owner relationships are not total. [It is partial participation]

But, for a car entity or a
bike entity to exist then there should be an owner related to them. That is, each
bike to be stored in Bike relation must have at least one owner. The same is
applicable for Car also. Hence, the participation of Car on Car_Owner and Bike on
Bike_Owner relationships are Total.

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